The Combat Phase contains 5 basic steps (up to seven total steps depending on whether there are creatures with Double/First/Last/Triple strike attacking). It officially begins once all players have passed priority in the First Main Phase.

Once you enter the Beginning of Combat Step (Rule 507) Toolcraft Exemplar's ability will trigger and go on the stack. Once it resolves, it will be a 3/2. You will then have priority and can activate the crew ability of your vehicle turning it into a creature.

The next step is the Declare Attackers Step (Rule 508) - the first part of this step is declaring which creatures you wish to use as attackers. So long as you have crewed your vehicle before entering this step and your vehicle is a creature at this time, you can use it as an attacker.

For future note, there is a Rules Q&A section of this forum designed for these types of questions. You can usually find some pretty quick answers there.

July 27, 2018
11:53 a.m.
Edited.

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